This invention relates to the alkylation of hydrocarbons to produce useful chemicals and motor fuel. This invention specifically relates to a process for producing motor fuel blending components by alkylating paraffins with olefins using a solid catalyst, which is regenerated in the presence of hydrogen.
Hydrocarbon alkylation is widely used in the petroleum refining and petrochemical industries to produce a variety of useful acyclic and cyclic hydrocarbon products that are consumed in motor fuel, plastics, detergent precursors, and petrochemical feedstocks. Alkylation comprises reacting an alkylation substrate feedstock such as isobutane and benzene with an alkylation agent feedstock such as C2-C22 olefins. For example, large amounts of paraffins for high-octane gasoline are produced by the alkylation of isobutane with butenes. In addition, valuable aromatic hydrocarbons including cumene, ethylbenzene, and C16-C22 linear alkylaromatics are produced in large amounts by alkylating benzene with olefins of the appropriate carbon number. The variety of feedstock alkylation substrates and alkylation agents and the passage of time has led to the development of a number of effective alkylation technologies which are employed in large scale commercial facilities. Much of the installed base of alkylation capacity uses liquid phase hydrofluoric acid, generally referred to as HF, as the catalyst.
FIGS. 1.4.3 and 1.4.4 of the book entitled Handbook of Petroleum Refining Processes, edited by Robert A. Meyers, Second Edition, McGraw-Hill, New York, 1997, show process flow diagrams of HF alkylation processes, including the product recovery facilities for recovering the hydrocarbons in the alkylation reactor effluent. Referring to these figures, the hydrocarbon phase, which contains alkylate, isobutane, some propane, and dissolved HF, flows from the acid settler, is preheated, and passes to a fractionation column, which is commonly called an xe2x80x9cisostripper.xe2x80x9d The hydrocarbon phase effluent from the reactor section enters at a feed tray near the top of the isostripper so that the isostripper consists mostly of a stripping section, except for a small rectification section on the top of the isostripper. The stripping section strips the more volatile HF, propane, and isobutane from the descending liquid alkylate, and product alkylate is recovered from the bottom of the isostripper. A bottom reboiler and one or more side reboilers add heat to the isostripper. When applicable, saturate field butane feed comprising isobutane and normal butane is fed to the stripping section of the isostripper at a tray below the reactor effluent feed tray, and any normal butane that may have entered the process is withdrawn from a sidedraw tray located below the field butane feed tray. Unreacted recycle isobutane is also withdrawn as a sidedraw, via a tray located between the reactor effluent and field butane feed trays. The rectification section reduces the concentration of the less volatile alkylate in the overhead vapor stream and thereby provides for efficient rejection of propane from the process. The overhead stream, which contains isobutane, propane, and HF, is condensed in an overhead condenser and collects in an overhead receiver. A drag stream of condensed overhead material undergoes further processing and separation in order to prevent an accumulation of propane in the process and to recycle isobutane and HF.
The use of HF in these motor fuel and detergent processes has a long record of highly dependable and safe operation. However, the potential damage from an unintentional release of any sizable quantity of HF and the need to safely dispose of some byproducts produced in the process has led to an increasing demand for alkylation process technology which does not employ liquid phase HF as the catalyst. U.S. Pat. No. 5,672,798, for example, discloses alkylating paraffinic hydrocarbons such as isobutane with olefinic hydrocarbons such as propylene or butenes in a fluidized riser-reactor using a solid catalyst. The effluent of the riser-reactor comprises the desired alkylate product, byproducts of the alkylation reaction, unreacted isobutane, and solid catalyst. The solid catalyst is separated and the remainder of the riser-reactor effluent passes to product recovery facilities.
Numerous solid alkylation catalysts have been described in the open literature. The previously cited U.S. Pat. No. 5,672,798 teaches a number of suitable solid catalysts that contain or have been treated with a Lewis acid, such as a large pore zeolite and a Lewis acid such as boron trifluoride and aluminum chloride, a large pore crystalline molecular sieve and a gaseous Lewis acid, a crystalline transition alumina treated with a Lewis acid, an acid washed silica treated with antimony pentafluroride, and a refractory inorganic oxide impregnated with a monovalent cation whose bound surface hydroxyl groups have been at least partially reacted with a Friedel-Crafts metal fluoride, chloride, or bromide.
These catalysts appear to suffer from slight but significant halogen loss rates when used at commercially useful alkylation reactor conditions. While some catalysts have a sufficiently useful halogen retention to allow the performance of alkylation, the gradual depletion of halogen results in a change in product composition and also requires the occasional replenishing of the halogen content of the catalyst. Some of the halogen loss is believed to be caused by the stripping of halogen from catalytically active sites of the catalyst by isobutane and also by the deposition on the catalytically active sites of heavy compounds. As used herein, the term xe2x80x9cheavy compoundsxe2x80x9d means molecules that have at least one carbon atom more than the number of carbon atoms than the highest number of carbon atoms of those molecules that are desired to be in the alkylate.
However, in addition to exhibiting halogen loss, these catalysts also seem to suffer from unacceptably high deactivation rates when employed at commercially feasible conditions. While some catalysts have a sufficiently useful lifetime to allow the performance of alkylation, the rapid change in activity results in a change in product composition and requires the periodic regeneration of the catalyst. Such periodic regeneration is typically accomplished by removing deactivated catalyst from the reaction zone, reactivating the catalyst in a separate zone, and returning the reactivated catalyst to the reaction zone. Some of the deactivation is believed to be caused by the deposition of heavy compounds on the catalytically active sites of the catalyst.
Continuous processes for alkylation that are not subject to periodic reaction zone stoppages or variation in the product stream composition are desirable, and the previously mentioned U.S. Pat. No. 5,672,798 describes such a process. In order to remove the heavy hydrocarbon deposits and at least partially restore the activity of the catalyst, U.S. Pat. No. 5,672,798 teaches contacting the catalyst within the process with hydrogen in two separate and simultaneous modes of regeneration: a mild liquid-phase washing and a hot vapor-phase stripping.
The hot vapor-phase stripping which is disclosed in U.S. Pat. No. 5,672,798 consists of contacting the catalyst with a vapor-phase gas stream at a temperature that is typically greater than that employed in the alkylation zone. Because the gas stream uses hydrogen and the contacting occurs at an elevated temperature, hot vapor-phase stripping, which is also referred to in U.S. Pat. No. 5,672,798 as xe2x80x9chydrogen strippingxe2x80x9d or xe2x80x9csevere regeneration.xe2x80x9d U.S. Pat. No. 5,627,798 teaches that the presence of some isobutane in the gas stream is desirable to increase the heat capacity of the gas and thereby to increase the catalyst heat-up rates. This hot hydrogen-isobutane stripping removes liquid phase hydrocarbons and deposits of heavy compounds from the catalyst and produces a vapor phase regeneration zone effluent stream. U.S. Pat. No. 5,672,798 teaches that this regeneration zone effluent stream is preferably first cooled sufficiently to condense substantially all of the hydrocarbons within the stream and then subjected to vapor-liquid phase separation. The recovered liquids pass to the products recovery facilities, and the hydrogen is recycled to the severe regeneration zone.
The mild liquid-phase washing which is disclosed in U.S. Pat. Nos. 5,310,713 and 5,672,798 comprises contacting the catalyst with a liquid-phase stream which is preferably the feed alkylation substrate (e.g., isobutane). This contacting generally occurs at a lower temperature than that of severe regeneration, and partly for this reason this contacting is often referred to as xe2x80x9cmild regeneration.xe2x80x9d U.S. Pat. Nos. 5,310,713 and 5,672,798 teach that hydrogen is preferably dissolved in this liquid-phase stream by a controlled addition up to the point of the stream containing the stoichiometrically required amount of hydrogen. These patents also teach that, for purposes of computing the stoichiometric requirement, the catalyst is analyzed in a laboratory for its heavy hydrocarbon deposit and the heavy hydrocarbon deposits are assumed to be composed of monoolefinic octenes. Some of this hydrogen is chemically consumed by saturating unsaturated hydrocarbons on the catalyst surface. In addition to reactivated catalyst, which is the desired product of the mild regeneration, a liquid-phase effluent is also recovered. This mild regeneration effluent usually contains hydrogen up to the point of saturation of hydrogen. The mild regeneration effluent combines with the riser-reactor effluent, and the combined effluents flow to the product recovery facilities.
The amount of hydrogen that is typically introduced into either the severe or mild regeneration zone is in excess of the amount that reacts with heavy hydrocarbon deposits in that zone, and therefore hydrogen is present in the severe regeneration effluent and/or the mild regeneration effluent. Because this hydrogen in these effluent(s) can still be useful in regenerating the catalyst, it is desirable to recycle this hydrogen to the regeneration zone(s). Therefore, methods are sought to recover and recycle hydrogen that is present in the regeneration effluent(s).
This invention is a paraffin-olefin alkylation process using a solid catalyst with a catalyst regeneration zone, in which an alkylation reactor effluent passes to an alkylate fractionation zone and a hydrogen-containing regeneration effluent passes to a hydrogen fractionation zone. While the alkylate fractionation zone recycles to the alkylation reactor compounds such as unreacted paraffinic feed or such as halogen-containing species to maintain the halogen content of the catalyst in the alkylation reactor, the hydrogen fractionation zone recycles molecular hydrogen to the regeneration zone to reactivate the catalyst. The hydrogen fractionation zone prevents molecular hydrogen from mixing with the reactor effluent, from entering the alkylate fractionation zone, and thus from being recycled to the alkylation reactor. By segregating molecular hydrogen in the regeneration effluent from the reactor effluent, the alkylate fractionation zone can in one embodiment of this invention produce a recycle stream comprising unreacted paraffinic feed or halogen-containing species that is substantially free of molecular hydrogen, that is, less than 500 wt-ppm molecular hydrogen. Therefore, the hydrogen fractionation zone maximizes the use of molecular hydrogen for regeneration and minimizes passing of molecular hydrogen to the alkylation reactor.
This invention is an improvement over prior art processes such as U.S. Pat. No. 5,672,798, which does not pass either the mild regeneration effluent or the severe regeneration effluent to a hydrogen fractionation zone, and therefore causes the olefin alkylating agent to be used very inefficiently. In the case of the mild regeneration effluent, U.S. Pat. No. 5,672,798 teaches combining the mild regeneration effluent with the riser-reactor effluent and passing the combined effluents to the product recovery facilities. Thus, the hydrogen chloride in the riser-reactor effluent inevitably becomes mixed with the molecular hydrogen in the regeneration effluent, and because the volatilities of molecular hydrogen and hydrogen chloride at commercially feasible fractionation conditions are relatively close so that molecular hydrogen and hydrogen chloride are difficult to separate from each other using the isostripper, the isostripper overhead stream contains both molecular hydrogen and hydrogen chloride. Therefore, recycling of the overhead stream to the inlet of the riser-reactor in order to replenish the chloride content of the catalyst would also recycle molecular hydrogen to the inlet of the riser-reactor. This has a detrimental effect on the alkylation performance, because molecular hydrogen is introduced at a point where unreacted olefin is present, which allows molecular hydrogen to saturate the olefin and thereby to render olefin ineffective as an alkylating agent. In contrast, by preventing molecular hydrogen from entering the alkylate fractionation zone, this invention recovers and recycles molecular hydrogen in the mild and/or severe regeneration effluents, without incurring the detrimental effect of loss of effective alkylating agent. In the case of the severe regeneration effluent, U.S. Pat. No. 5,672,798 teaches passing the severe regeneration effluent to a vapor-liquid separator, separating a heavy hydrocarbon liquid phase from the vapor phase, and passing the liquid phase to conventional product recovery facilities. It has now been recognized, however, that a significant portion of the molecular hydrogen that enters the vapor-liquid separator with the severe regeneration effluent exits the vapor-liquid separator with the liquid phase, rather than the vapor phase, because molecular hydrogen is dissolved in, entrained in, or otherwise contained in or carried with the liquid phase. Therefore, despite the use of a vapor-liquid separator, the process of U.S. Pat. No. 5,672,798 nevertheless passes significant and unacceptable quantities of molecular hydrogen to the isostripper and in turn to the riser-reactor.
Another advantage of this invention over the process in U.S. Pat. No. 5,672,798 is a reduction in the capital cost and operating costs of the isostripper. This invention not only prevents the mixture of molecular hydrogen and hydrogen chloride but also that of molecular hydrogen and the alkylation substrate (e.g., isobutane), in the isostripper. Isobutane is generally introduced in stoichiometric excess at alkylation conditions, is therefore usually present in the alkylation reaction effluent, and is accordingly recycled by the isostripper to the alkylation reaction zone. Any molecular hydrogen entering the isostripper would thus have to be separated not only from hydrogen chloride but also from the isobutane. However, this latter separation requires a significant increase in the number of trays, especially in the upper section of the isostripper, as well as a significant increase in the reboiler duty. By using a hydrogen fractionation zone, this invention avoids the costs associated with adding these additional trays and providing additional heat utilities.
Therefore, a broad objective of this invention is to alkylate paraffins with olefins using a solid catalyst that is regenerated in the presence of hydrogen. Another broad objective of this invention is to alkylate paraffins with olefins using a regenerable solid catalyst in which hydrogen is used efficiently for regeneration while avoiding any detrimental reaction of hydrogen and olefins. This invention is well-suited for processes that use a solid catalyst and in which halogen is used to maintain catalyst performance because this invention allows for recycling halogens in order to replenish the catalyst halogen content.
Accordingly, in a broad embodiment, this invention is an alkylation process comprising passing a first feed stream comprising a paraffinic alkylation substrate and a second feed stream comprising an olefinic alkylating agent to an alkylation reaction zone. The alkylation reaction zone is operated at alkylation conditions selected to react the paraffinic alkylation substrate and the olefinic alkylating agent in the presence of a solid catalyst to produce alkylate. The alkylation conditions are also sufficient to deposit heavy compounds on the solid catalyst in the alkylation reaction zone. An alkylation reaction effluent comprising the alkylate and the paraffinic alkylation substrate is withdrawn from the alkylation reaction zone. A first catalyst stream comprising solid catalyst having heavy compounds deposited thereon is withdrawn from the alkylation reaction zone. At least a portion of the first catalyst stream passes to a first regeneration zone. The solid catalyst having heavy compounds deposited thereon is contacted with molecular hydrogen in the first regeneration zone at first regeneration conditions selected to remove at least a portion of the heavy compounds from the solid catalyst having heavy compounds deposited thereon and to at least partially regenerate the solid catalyst having heavy compounds deposited thereon. A second catalyst stream comprising at least partially regenerated solid catalyst is withdrawn from the first regeneration zone. At least a portion of the second catalyst stream passes to the alkylation reaction zone. A first regeneration effluent comprising molecular hydrogen and the heavy compounds is withdrawn from the first regeneration zone. At least a portion of the first regeneration effluent passes to a hydrogen fractionation zone. A hydrogen-enriched stream having a first concentration of molecular hydrogen is recovered from the hydrogen fractionation zone. A hydrogen-depleted stream comprising the heavy compounds and having a second concentration of molecular hydrogen that is less than the first concentration of molecular hydrogen is also recovered from the hydrogen fractionation zone. At least a portion of the hydrogen-enriched stream passes to the first regeneration zone. At least a portion of the alkylation reaction effluent and at least a portion of the hydrogen-depleted stream passes to an alkylate fractionation zone. A recycle stream comprising the paraffinic alkylation substrate is withdrawn from the alkylate fractionation zone. The first feed stream is formed from at least a portion of the recycle stream. The alkylate is recovered from the alkylate fractionation zone.
Chapters 1.4 and 1.5 of the book entitled Handbook of Petroleum Refining Processes, edited by Robert A. Meyers, Second Edition, McGraw-Hill, New York, 1997 describe HF alkylation processes for motor fuel production and detergent manufacture.
U.S. Pat. No. 5,489,732 (Zhang et al.); U.S. Pat. No. 5,672,798 (Zhang et al.); and U.S. Pat. No. 5,675,048 (Zhang et al.) disclose alkylation processes that use a solid catalyst which is regenerated by a xe2x80x9cmild,xe2x80x9d low-temperature, liquid phase washing and by a xe2x80x9csevere,xe2x80x9d hot vapor phase hydrogen stripping operation. The teachings of U.S. Pat. Nos. 5,489,732; 5,672,798; and 5,675,048 are incorporated herein by reference.
U.S. Pat. No. 5,310,713 (Kojima et al.) discloses a solid catalyst alkylation process wherein the reaction mixture is a liquid phase, the catalyst is treated with hydrogen, and the hydrogen treatment may be effected with either liquid-free catalysts or in the presence of liquid isobutane and a chloride source. The teachings of U.S. Pat. No. 5,310,713 are incorporated herein by reference.